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Dynamic "moth-ematics"

Bill Nelson

Photo: Bernard Clark

Follow the trail Professor Bill Nelson’s research is blazing and you’ll end up in a room in the basement of the Biosciences Complex. There, after donning a lab coat and elasticized booties that slip on over your street shoes, he guides you into a screened-off area filled with boxes created out of hard pink insulation, resting on industrial shelving, each connected by hoses to a noisy cooling system sitting in the corner. Those boxes, each kept at a separate temperature, house Japanese tea tortrix moths at the different stages of their life cycle – egg, larva, pupa, and adult.

“What I do, if you want to put a name on it,” Nelson says, “is physiologically-structured population biology – in my case by bringing together mathematical and experimental biology.” Simply, explains Nelson, a member of the Queen’s Department of Biology, most traditional ecology focuses on total populations. It looks at predator and prey relationships, the rise and fall of entire populations, but never generally pays much attention to the individual members of the population under study. Nelson, by contrast, focuses on the individuals, in particular where they are in their life cycle, and how this generates much bigger population changes. Using data gleaned by studying the life stages of individual members of a species in the lab, he creates mathematical models that can be used to provide insights into the behaviour of larger animal populations in the natural environment. His goal is to understand the “underlying fundamental principles behind population dynamics.”

Nelson’s initial insights into the importance of the individual in these dynamics came from his work on the zooplankton known as Daphnia (a.k.a. the water flea). This incredibly common plant-eating microorganism, Nelson calls them “the cows of the lakes,” is found in abundance in freshwater everywhere.

Considered at the population level, and following standard ecological models, the expectation would be that numbers of Daphnia in any population should oscillate wildly as their food supply increases and decreases. “In those systems,” says Nelson, “you expect crazy cycles. But you never see them.” Instead, Nelson found what altered was the length of time juveniles took to become adults. The less food, the longer each member took to reach maturity, which prevented the expected wild cycles.

Nelson continues to work with Daphnia, and has also expanded his research to examine the importance of the life cycle in bean weevils – drawing him away from his initial work as a freshwater biologist to concentrating on terrestrial insects. His goal, always, has been to “push his research,” and take it in new directions.

Five years ago, he got his chance. For more than 50 years, Japanese scientists have been conducting population counts on a moth known as the tea tortrix at one tea plantation in Japan, counting its numbers every five days. “Fifty years of data,” says Nelson. “I had never come across a system like this before.”

The tea tortrix is known as an “outbreak species” – the moth’s population is liable to erupt suddenly, four times in some years, five in others. This is a particularly undesirable behaviour in what is, after all, a pest species, and over the years, tea planters had tried a wide range of methods to control these outbreaks. “Everything,” Nelson says, “from DDT, to mate disruption pheromones, bacteria, viruses – none of these control strategies had any effect on population dynamics.”

Scientists have long known that changing temperatures have an effect on the individual development of insects. Could the effect of temperature explain the tea tortrix’s outbreaks? The challenge for Nelson and the scientists he has teamed with – Ottar N. Bjørnstad at Penn State University and Japanese scientist Takehiko Yamanaka – was to replicate natural variations in temperature in the laboratory, hence those pink boxes. Each is maintained at a different temperature to allow Nelson and his students to observe the effects of both constant and changing temperatures on small moth populations.

Nelson’s research suggests that the effects of temperature on individual development lay behind the outbreaks. The population of moths would be stable until “you hit 12 degrees,” says Nelson. “Then boom,” the moth’s population erupts. From the data they had collected in the lab, generally studying how fast larvae matured, Nelson was able to create a mathematical model that predicted the outbreaks. When this model was compared to the 50 years of data, the correspondence between them was immediate and obvious.

Understanding the role of temperature in a pest species can help us to control it. It may also point to future problems caused by climate change – in a warmer world, outbreak species such as the tea tortrix moth may be an even greater problem. For Nelson, however, there are fundamental questions still to be answered. Why does the moth have these outbreaks? What is the evolutionary explanation? “I plan to spend the next five years trying to find out.”